Currently, crystalline silicon has the largest market share in the photovoltaics (PV) industry, accounting for over 85% of the overall PV market share. The relatively high efficiencies associated with crystalline solar cells compared to most thin-film technologies, combined with the abundance as well as the environmentally benign and non-toxic nature of the material, garner appeal for continued use and advancement. Going to thinner crystalline silicon solar cells is understood to be one of the most potent ways to reduce PV manufacturing cost and the resulting Levelized Cost of Electricity (LCOE) because of the relatively high material cost of crystalline silicon wafers used in solar cells as a fraction of the total PV module cost (being on the order of 50% of the total PV module cost). And while deposition of epitaxial silicon film has been in use in the semiconductor industry (for applications such as bipolar chips and smart power devices), the high-productivity production of epitaxial crystalline silicon layers, including the production of monocrystalline silicon utilizing a single or double sided epitaxial deposition process, for use in solar cells or other applications at high production volumes and at a low cost has posed many challenges.
In the monocrystalline silicon epitaxy (epi) process, the film is deposited using a mixture of a silicon source gas, such as trichlorosilane (TCS), and/or silicon tetrachloride, and hydrogen at temperatures typically ranging between 1050° C. to 1250° C. Since deposition may happen on any exposed heated surface after the precursor gases are heated, it is advantageous to reduce the chamber area allocated to gas heating as much as possible. However, as the number of wafers in a conventional batch epi chamber increases so does the gas flow rate and the total heat rate. And one problem with increasing the heating area to accommodate the total heat rate for the higher gas flow rate is that this exposes a larger portion of the chamber to unwanted or parasitic deposition.
Furthermore, it is desirable to reduce the heated epi chamber areas (parasitic deposition regions) that are not covered by the target silicon substrates. The lower the ratio of uncovered to covered areas, the higher the effective source gas utilization and, potentially in most cases, the less the maintenance cost of susceptor cleaning. For this and other productivity related reasons, it is desirable to increase the number of wafers (or the wafer batch size) within a given deposition chamber. Similarly, and for other reasons such as thermal budget considerations and substantially increasing the manufacturing productivity, it may be desirable to grow epi films on both sides of the silicon substrate—such as applications where epi films that are deposited on both sides of a reusable crystalline semiconductor template are harvested and lifted off for solar cell fabrication at essentially double the harvesting rate of the single-sided templates.
One of the most promising technologies to achieve high solar cell efficiency at low silicon usage is the use of deposition of silicon as a thin film or foil between a fraction of 1 micron and 100 micron (μm) thickness on carrier wafers (templates). These templates have a designated weak release or separation (or cleavage) layer or layer system, which may be a porous semiconductor or specifically porous silicon layer, for the subsequent removal of said deposited thin semiconductor film or foil, which may require the use of a reinforcement layer to prevent mechanical breakage due to the thin (≦100 μm) and large (≧100 cm2) substrate sizes of the thin film or foil. Thus, at least a portion of such porous layers are used as designated weak layers along which the deposited epi film may be lifted off from the substrate that it has been deposited on. However, current deposition equipment and processes are too costly and complex for large scale high volume deposition of epitaxial silicon thin film.
Another complication in high volume manufacturing of epi films is the amount of power required for each tool to heat the substrates and the gas to the necessary process temperature. Because the peak power required for each tool often reaches hundreds of kilowatts or even megawatts, individual or multi-tool start-ups can create huge electrical surges and sags in the plant if not managed.
Yet another concern for growing high quality epi film on a silicon substrate is the presence of moisture adsorbed or trapped in the substrate. This is particularly important if the substrate surface is made porous or contains several anodically etched porous layers, as is the case, for example, with thin monocrystalline silicon epitaxy films intended for subsequent lifting from the base material (reusable template) through the use of a sacrificial porous layer. This problem is confounded when epi films are deposited on bi-layer or multi-layer (or graded porosity) porous silicon structures.
Designing highly productive equipment requires a good understanding of the process requirements and reflecting those requirements in the equipment architecture. High manufacturing yield of thin film substrates requires a robust process and reliable deposition equipment. Thus, a highly productive, reliable, and efficient reactor is essential for the high-throughput production of low-cost, high-efficiency solar cells.